Problem: Which of these numbers is prime? ${11,\ 51,\ 55,\ 63,\ 84}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 51 are 1, 3, 17, and 51. The factors of 55 are 1, 5, 11, and 55. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 84 are 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, and 84. Thus, 11 is a prime number.